The invention relates to an a.c. voltage regulator employing a bilateral thyristor, and more particularly, to a phase controlled a.c. voltage regulator which employs a three terminal, bilateral thyristor. A conventional phase controlled a.c. voltage regulator which uses a three terminal, bilateral thyristor is shown in FIG. 1, in which the opposite terminals 1a, 1b of an a.c. source 1 is connected in a series circuit comprising a load 2 and a three terminal, bilateral thyristor 3. A time constant circuit comprising a series combination of a variable resistor 7 and a capacitor 8 is associated with the thyristor, with one end of the resistor 7 connected with a second anode terminal 4 of the thyristor 3 and one terminal of the capacitor 8 with a first anode terminal 5 of the thyristor 3. The junction between the resistor 7 and the capacitor 8 is connected with a gate terminal 6 of the thyristor through a bilateral diode 10. The three terminal, bilateral thyristor may comprise TRIAC (trademark of General Electric Company), for example, and the bilateral diode 10 may comprise a DIAC (trademark of General Electric Company).
In the conventinal regulator circuit shown, the bilateral thyristor 3 is initially non-conductive, so that the capacitor 8 is charged through the load 2 and the variable resistor 7 connected in series. As the terminal voltage of the capacitor 8 reaches a breakover voltage of the bilateral diode 10, the latter conducts, applying a trigger pulse to the gate terminal 6 in synchronism with each half cycle of the source frequency. Thereupon, the thyristor conducts, whereby current flows from the source 1 through the load 2 and the thyristor 3. The phase angle at which the thyristor 3 begins its conduction can be controlled by changing the time constant of resistor 7 and capacitor 8, thereby changing the phase angle of the trigger pulse which is produced during each one-half cycle of the alternating current. In this manner, the average voltage or the effective value thereof across the load 2 can be regulated.
However, with the described circuit, when the voltage of the source 1 rises, a charging current to the capacitor 8 increases, so that the thyristor 3 initiates its conduction earlier in time, thus increasing the effective voltage across the load 2. Thus, a voltage fluctuation of the source 1 is amplified to cause an even greater variation in the effective voltage of the load 2. In one experiment in which a source 1 having a voltage of 100 volts is used and the effective voltage across the load 2 is adjusted to 30 volts, it is found that a fluctuation of the source voltage of .+-.10% from the nominal value resulted in a change of .+-.100% in the effective voltage across the load 2 from the adjusted value of 30 volts. In addition, with the circuit described, if the direction of current flow from the source 1 is reversed before the terminal voltage of the capacitor 8 reaches the breakover voltage of the bilateral diode 10, the charge on the capacitor 8 cannot have time to discharge before it is charged in the opposite direction, preventing the voltage across the capacitor 8 from reaching the breakover voltage of the diode 10. This means that the circuit fails to adjust effective voltage across the load 2 over a range beginning from zero volts. In one experiment using a source voltage of 100 volts, it is found that when it is attempted to increase the effective voltage from zero volts, it jumps from zero volt to 25 volts stepwise. When reducing the effective voltage from a level of 25 volts, it also jumps from 10 volts to zero volt.